Synthesis of nickel and cobalt oxide nanoparticles by pulsed underwater spark discharges

Electrical discharges in liquids are considered an efficient and ecological technique of nanoparticle synthesis via controlled erosion of electrodes. Herein, we use spark discharges between Co–Co, Ni–Ni, Co–Ni, or Ni–Co electrodes immersed in distilled water to synthesize Co and/or Ni nanoparticles, as well as their oxides. When mixed electrodes are used (Co–Ni or Ni–Co), both Co and Ni nanoparticles are produced, and the major species is dictated by the nature of the anode pin. The characteristics of nanoparticles synthesized under varying conditions of pulse width (100 and 500 ns) and voltage amplitude (5 and 20 kV) are analyzed by transmission electron microscopy. Within the investigated discharge conditions, it is not possible to produce Co–Ni nanoalloys; however, core–shell nanoparticles are observed among the Ni and Co nanoparticles. Finally, the direct optical bandgaps of the nanomaterials are determined using UV-visible absorption spectroscopy.